Current layers:: Influence of the finite size and non-uniform current distribution

One of the key parameters of the auroral regions is the currents which flow parallel to the magnetic field and transfer the energy and momentum between the ionosphere and the external regions of the magnetosphere. Satellite magnetic field measurements provide information about such currents. The commonly adopted technique to deduce the current characteristics from the magnetic perturbations is based on the approximation of current sheet of three dimensions, two of them, i.e. along the magnetic field and in a perpendicular direction, are infinite. This approach is valid when the magnetic perturbations are observed far from the current edges, which is not always the case. In this paper, we compare the infinite sheet approximation with the solution for the current layer of finite transverse sizes and discuss the influence of the current edges on the deduced parameters. In addition, the current distribution inside the layer is rarely uniform, but comprises filamentary structures with typical sizes that correspond to the characteristic scales of the processes which originate them. Both, the filamentary currents and the current sheet edges, create a component of the magnetic perturbation normal to the current sheet, which is absent in the model of the infinite uniform current layer. The discussion is illustrated with simultaneous observations of a current double-layer made at the altitude of 860 km by the magnetometers on board the Orsted satellite and at similar to 300 km by the coherent SuperDARN radars. (c) 2005 Elsevier Ltd. All rights reserved.